Comprehensive Study Guide on Acids, Alkalis, and Neutralisation

Introduction to Acids and Alkalis

Etymology and Definitions: * Acid: The word is derived from the Latin word acidus, which means "sour." Acids are substances that dissolve in water to form acidic solutions. * Alkali: The word is derived from the Arabic word alqali, which means "ashes from the burning of saltwort plants."
Natural Occurrences of Acids: * Citric acid: Found in citrus fruits such as oranges, limes, and lemons. * Malic acid: Found in apples. * Tartaric acid: Found in grapes. * Lactic acid: Found in yoghurt. * Acetic acid: Found in vinegar. * Oxalic acid: Found in tomatoes. * Carbonic acid: Found in fizzy (carbonated) drinks. * Formic acid: Found in most ants. * Tannic acid: Found in tea.

Properties and Safety of Acids

Physical and Chemical Properties: * Taste: Acids have a sour taste (e.g., lemons). * Litmus Test: Acids change the colour of blue litmus paper to red. * Electrical Conductivity: Acids are good electrical conductors. For example, sulphuric acid ( $H_{2}SO_{4}$ ) is used in car batteries because it conducts electricity well. * Corrosivity: Acids react with certain materials causing corrosion. In car batteries, acid reacts with metal terminals to form flaky blue, white, or green substances.
Chemical Reactions of Acids: 1. Reaction with Carbonates: Acids react with carbonates to produce salt, water, and carbon dioxide gas ( $CO_{2}$ ). The gas is tested by passing it through limewater, which turns milky. * $\text{Calcium carbonate} + \text{Hydrochloric acid} \rightarrow \text{Calcium chloride} + \text{Water} + \text{Carbon dioxide}$ 2. Reaction with Alkalis: An acid reacts with an alkali to produce salt and water (Neutralisation). 3. Reaction with Metals: Acids react with certain metals to produce salt and hydrogen gas ( $H_{2}$ ). The hydrogen gas is identified by a "pop" sound when tested with a lighted wooden splinter. * $\text{Magnesium} + \text{Hydrochloric acid} \rightarrow \text{Magnesium chloride} + \text{Hydrogen}$
Laboratory Safety and Handling: * Common lab acids include hydrochloric acid ( $HCl$ ), nitric acid ( $HNO_{3}$ ), sulphuric acid ( $H_{2}SO_{4}$ ), and ethanoic acid. * Precautions: * Do not touch or taste acids without permission. * If swallowed, spit out immediately and rinse with plenty of water. * If skin contact occurs, wash with lots of water and inform a teacher. * Wear safety goggles and gloves. * Dilution Rule: Always add acid to water, never add water to acid. * Hazard Symbols: Acids are labeled with a universal corrosive hazard symbol to warn users quickly regardless of language.

Properties of Alkalis

Physical and Chemical Properties: * Taste: Alkalis taste bitter. For example, soap bubbles taste bitter if they accidentally enter the mouth. * Feel: Alkalis feel soapy and slippery to the touch (e.g., shower gel). * Litmus Test: Alkalis change the colour of red litmus paper to blue. * Electrical Conductivity: Alkalis are good electrical conductors. Potassium hydroxide ( $KOH$ ) is used in alkaline batteries for this reason. * Corrosivity: Strong alkalis like sodium hydroxide ( $NaOH$ ) and potassium hydroxide ( $KOH$ ) are extremely corrosive.
Chemical Reactions of Alkalis: * Reaction with Acids: Alkalis react with dilute acids to produce salt and water. * $\text{Potassium hydroxide} + \text{Nitric acid} \rightarrow \text{Potassium nitrate} + \text{Water}$

Summary Comparison: Acids vs. Alkalis

Similarities: * Most strong acids and alkalis are corrosive. * Both change the colour of litmus paper. * Both are electrolytes (conductors of electricity).
Differences: * Feel: Acids are not slippery; Alkalis are slippery. * Taste: Acids are sour; Alkalis are bitter. * Blue Litmus: Acids turn it red; Alkalis cause no change. * Red Litmus: Acids cause no change; Alkalis turn it blue. * pH value: Acids are $pH\,0 - pH\,6$ ; Alkalis are $pH\,8 - pH\,14$ . * Metal Reaction: Acids release hydrogen gas; Alkalis generally have no reaction with most metals. * Ionic Composition: Acids are composed of hydrogen ( $H^{+}$ ) ions; Alkalis are composed of hydroxide ( $OH^{-}$ ) ions. * Strength: The strength of an acid depends on the concentration of $H^{+}$ ions; for alkalis, it depends on the concentration of $OH^{-}$ ions.

The pH Scale and Indicators

pH Scale Fundamentals: * The scale ranges from $0$ to $14$ . * Acidic: $pH < 7$ (Lower values mean stronger acidity). * Neutral: $pH = 7$ . * Alkaline: $pH > 7$ (Higher values mean stronger alkalinity).
Logarithmic Nature of pH: * A change of one integer value represents a tenfold ( $10 \times$ ) change in concentration. * Example 1: A solution with $pH\,2$ is $10$ times more acidic than a solution with $pH\,3$ . * Example 2: A solution with $pH\,11$ is $10$ times more alkaline than a solution with $pH\,10$ . * Calculation Challenge: A solution with $pH\,1$ is $100$ times more acidic than a solution with $pH\,3$ (since $10 \times 10 = 100$ ).
Indicators: * Universal Indicator: Displays a range of colours to indicate the strength of an acid or alkali. It turns green in neutral solutions ( $pH\,7$ ). * Other Indicators: Red and blue litmus paper, methyl orange, and phenolphthalein. * pH Meter: A device used to get a more precise digital reading of pH compared to the subjective colour matching of indicators.

Neutralisation

Definition: The reaction between the right amounts of an acid and an alkali to produce a neutral solution containing salt and water with a $pH$ of $7$ .
General Equation: $Acid + Alkali \rightarrow Salt + Water$
Method: Neutralisation is typically carried out through a process known as titration.
Reaction Examples: 1. $\text{Hydrochloric acid} + \text{Sodium hydroxide} \rightarrow \text{Sodium chloride (salt)} + \text{Water}$ 2. $\text{Nitric acid} + \text{Potassium hydroxide} \rightarrow \text{Potassium nitrate (salt)} + \text{Water}$ 3. $\text{Sulphuric acid} + \text{Potassium hydroxide} \rightarrow \text{Potassium sulphate (salt)} + \text{Water}$

Daily Life Applications of Neutralisation

Soil Treatment: * Crops grow best in soil that is not too acidic. Excessive use of fertilisers and pesticides increases soil acidity. * Neutralising agents added to soil: Quick lime (calcium oxide, $CaO$ ) or slaked lime (calcium hydroxide, $Ca(OH)_{2}$ ).
Stomach Indigestion: * The stomach produces hydrochloric acid ( $HCl$ ) to help digestion and kill germs. Excess acid causes discomfort/indigestion. * Antacids: Alkaline tablets (e.g., magnesium hydroxide or sodium bicarbonate) are used to neutralise the excess acid and relieve pain.
Reducing Air Pollution (Scrubbing): * Burning fossil fuels releases acidic sulphur dioxide ( $SO_{2}$ ) gas, which causes acid rain. * Scrubbing: Power stations use alkaline substances like limestone (calcium carbonate, $CaCO_{3}$ ) or sodium hydroxide to neutralise $SO_{2}$ before it is released through chimneys.
Common Neutralising Substances: * Sodium bicarbonate. * Magnesium hydroxide. * Calcium oxide. * Calcium carbonate.

Environmental Issues: Acid Rain

Definition: Rain that is unusually acidic (low pH) due to atmospheric pollutants.
Formation Process: 1. Burning fossil fuels in power plants, vehicles, and oil refineries releases sulphur dioxide ( $SO_{2}$ ) and nitrogen dioxide ( $NO_{2}$ ). 2. Pollutants rise into the atmosphere and react with water, oxygen, and other chemicals. 3. Sulphuric acid ( $H_{2}SO_{4}$ ) and nitric acid ( $HNO_{3}$ ) are formed. 4. These acids mix with rainwater and fall to Earth, entering water systems and soil.
Detrimental Effects: * Environment: Harms soil, plants, insects, and aquatic organisms. * Infrastructure: Corrodes steel structures (e.g., bridges) and deteriorates/erodes limestone buildings and sculptures, leading to the loss of carved details. * Health: Can negatively affect human health.